Class-A drug taken up to escape (3-6)
I believe the answer is:
top-flight
'class-a' is the definition.
(I've seen this before)
'drug taken up to escape' is the wordplay.
'drug' becomes 'pot' (slang term for cannabis).
'taken up' says the letters should be written in reverse.
'to escape' becomes 'flight' (synonyms).
'pot' in reverse letter order is 'top'.
'top'+'flight'='TOP-FLIGHT'
(Other definitions for top-flight that I've seen before include "Of the highest class" , "Superior" , "Highest level" , "Excellent -- best possible" .)